In a typical lead acid battery, the cell is composed of a lead negative electrode and a lead oxide positive electrode, together with a sulfuric acid electrolyte. When the electrodes are positioned on opposite sides of an electrically conductive substrate, the assembly is referred to as a bipolar plate. The electrical interconnection between cells in a bipolar battery is provided by the substrate whereas in a conventional monopolar battery the electrical interconnection is made by intercell welding of lead straps.
A bipolar battery has advantages in power capability, volume and weight over a conventional monopolar design. The advantages are achieved primarily because of the shorter current path, from which the battery internal resistance is lowered, and the elimination of the lead straps and the use of lightweight substrates, both of which lower the weight of the battery.
In practice, a bipolar battery consists of a stack of generally rectangular non-metallic frames formed of thermoplastic material, including a pair of end frames and a series of central frames. Each central frame is provided with an opening and a bipolar plate is mounted in each opening with opposed surfaces of the frame projecting outwardly beyond the bipolar plate. The frames are secured in contiguous flatwise relation and a separator layer, formed of a material such as glass mat, separates the bipolar plates of the frames. The sealed stack is subsequently filled with an electrolyte, such as sulfuric acid.
A problem encountered in the past with bipolar batteries has been leakage of the acid between the frames, due to a defective seal between the frames. Acid wetting between frames results in shorting between cells and causes high self-discharge of the battery. The battery service life decreases when self-discharge is present due to the poor seals between cells.
In the past, the thermoplastic frames of the bipolar battery have been joined by heat sealing techniques in which a heated platen, at a temperature of above 700.degree. F., is brought into contact with the surfaces of the frames to fuse or melt the surfaces. The fused surfaces are then brought into contact to provide the connection. the conventional bipolar battery frame is relatively thin, having a thickness in the neighborhood of about 0.050 inch, and the thermoplastic material is normally reinforced with glass or graphite fibers. When the thermoplastic material is heated through use of a platen, the fibrous reinforcement can be exposed with the result that an inadequate bond is achieved between the two thermoplastic frames. In addition, as the frames are relatively thin, the high temperature platen heating can cause warpage of the frames.
A further problem encountered during platen heating is that when the heated platen is withdrawn from the thermoplastic frame, molten plastic may pull away from the frame in the form of fine strings. The strings will chill or solidify before the surface of the frame, and as a result, will not provide an adequate bond. The chilled strings can cause leak paths between the frames and can also interfere with the fill channels which are formed in the surface of the frame and are employed when the battery is filled with acid. Therefore, there has been a need for a procedure for joining the thermoplastic bipolar battery frames which will provide a positive leakproof connection.